<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>hello world</title>
    <script src="js/angular.js"></script>
</head>
<body>
<div ng-app="myApp" >
    <div ng-controller="myCtrl">
        <input type="text" ng-model="txt_value">
        value:{{content}}
    </div>

</div>

<script>
    var app = angular.module('myApp', []);
    app.controller('myCtrl', function($scope,$parse) {
        $scope.$watch('txt_value',function (newvalue,oldvalue,scope) {
            if(newvalue!==oldvalue){
                var parseFun=$parse('txt_value');
                $scope.content=parseFun(scope);
            }
        })
    });

</script>
</body>
</html>